In recent years, from the viewpoint of soaring oil prices and exhaustion of petroleum estimated in the near future, deep oil wells which have not been searched and oil wells, gas wells, and the like in severe corrosive environments at so-called “sour” environments have been actively developed. In general, such oil wells and gas wells have very large depths and the atmospheres thereof are severely corrosive environments containing CO2, Cl− and, furthermore, H2S at high temperatures. Oil country tubular goods used in such environments are required to include materials having high strength and excellent corrosion resistance (carbon dioxide gas corrosion resistance, sulfide stress corrosion cracking resistance, and sulfide stress cracking resistance) in combination.
In oil wells and gas wells in environments containing carbon dioxide gas (CO2), chlorine ions (Cl−), and the like, in many cases, 13% Cr martensitic stainless steel tubes or pipes have been employed as oil country tubular goods used for development drilling. In addition, recently, use of improved version 13% Cr martensitic stainless steel has been spread, where C in the component system of 13% Cr martensitic stainless steel is reduced and Ni, Mo, and the like are increased.
For example, Japanese Unexamined Patent Application Publication No. 10-1755 describes an improved version 13% Cr martensitic stainless steel (steel tube or pipe), where the corrosion resistance of the 13% Cr martensitic stainless steel (steel tube or pipe) is improved. The stainless steel (steel tube or pipe) described in Japanese Unexamined Patent Application Publication No. 10-1755 is a martensitic stainless steel having excellent corrosion resistance and excellent sulfide stress corrosion cracking resistance, wherein in the composition of martensitic stainless steel containing 10% to 15% of Cr, C is limited to 0.005% to 0.05%, Ni: 4.0% or more and Cu: 0.5% to 3% are added in combination, 1.0% to 3.0% of Mo is further added, and Nieq is adjusted to −10 or more, and the microstructure is composed of a tempered martensite phase, a martensite phase, and a residual austenite phase, while a total fraction of tempered residual austenite phase and martensite phase is 60% to 90%. It is mentioned that the corrosion resistance and the sulfide stress corrosion cracking resistance are thereby improved in wet carbon dioxide gas environments and in wet hydrogen sulfide environments.
Also, oil wells in corrosive environments at higher temperatures (high temperatures up to 200° C.) have been recently developed. However, there is a problem that predetermined corrosion resistance cannot be stably sufficiently ensured in such high-temperature corrosive environments by the technology described in Japanese Unexamined Patent Application Publication No. 10-1755.
Then, oil country tubular or pipy goods which can be used in such high-temperature corrosive environments and which have excellent corrosion resistance and excellent sulfide stress corrosion cracking resistance have been desired and various martensitic stainless steel tubes or pipes have been proposed.
For example, Japanese Unexamined Patent Application Publication No. 2005-336595 describes a high-strength stainless steel tube or pipe, which has a composition containing, on a percent by mass basis, C: 0.005% to 0.05%, Si: 0.05% to 0.5%, Mn: 0.2% to 1.8%, Cr: 15.5% to 18%, Ni: 1.5% to 5%, Mo: 1% to 3.5%, V: 0.02% to 0.2%, N: 0.01% to 0.15%, and O: 0.006% or less such that Cr, Ni, Mo, Cu, and C satisfy a specific relational equation and Cr, Mo, Si, C, Mn, Ni, Cu, and N satisfy a specific relational equation, which has a microstructure containing a martensite phase as a basic phase and 10% to 60% of ferrite phase on a volume fraction basis or a microstructure further containing 30% or more of austenite phase, and which has excellent corrosion resistance. It is mentioned that a high-strength and, furthermore, high-toughness stainless steel tube or pipe for oil country tubular goods can be thereby stably produced having sufficient corrosion resistance even in severe corrosive environments containing CO2 and Cl− at high temperatures of 200° C. or higher.
Also, Japanese Unexamined Patent Application Publication No. 2008-81793 describes a high-strength stainless steel tube or pipe for oil country tubular goods having high toughness and excellent corrosion resistance. According to the technology described in Japanese Unexamined Patent Application Publication No. 2008-81793, the steel tube or pipe has a composition containing, on a percent by mass basis, C: 0.04% or less, Si: 0.50% or less, Mn: 0.20% to 1.80%, Cr: 15.5% to 17.5%, Ni: 2.5% to 5.5%, V: 0.20% or less, Mo: 1.5% to 3.5%, W: 0.50% to 3.0%, Al: 0.05% or less, N: 0.15% or less, and O: 0.006% or less such that Cr, Mo, W, and C satisfy a specific relational equation, Cr, Mo, W, Si, C, Mn, Cu, Ni, and N satisfy a specific relational equation, and Mo and W further satisfy a specific relational equation and has a microstructure containing a martensite phase as a basic phase and 10% to 50% of ferrite phase on a volume fraction basis. It is mentioned that a high-strength stainless steel tube or pipe for oil country tubular goods can be thereby stably produced having sufficient corrosion resistance even in severe corrosive environments containing CO2, Cl− and, furthermore, H2S at high temperatures.
Also, International Publication No. WO 2010/050519 describes a high-strength stainless steel tube or pipe having excellent sulfide stress cracking resistance and excellent high-temperature carbon dioxide gas corrosion resistance. According to International Publication No. WO 2010/050519, the steel tube or pipe has a composition containing, on a percent by mass basis, C: 0.05% or less, Si: 1.0% or less, Cr: more than 16% and 18% or less, Mo: more than 2% and 3% or less, Cu: 1% to 3.5%, Ni: 3% or more and less than 5%, and Al: 0.001% to 0.1% and containing Mn and N in such a way as to satisfy a specific relational equation in a region of Mn: 1% or less and N: 0.05% or less and has a microstructure containing a martensite phase as a basic phase, 10% to 40% of ferrite phase on a volume fraction basis, and 10% or less of residual austenite phase on a volume fraction basis. It is mentioned that a high-strength stainless steel tube or pipe is thereby produced further having sufficient corrosion resistance even in carbon dioxide gas environments at a high temperature of 200° C., having sufficient sulfide stress corrosion cracking resistance even when the environmental gas temperature is lowered, and having excellent corrosion resistance.
Also, International Publication No. WO 2010/134498 describes a stainless steel tube or pipe for oil country tubular goods having a composition containing, on a percent by mass basis, C: 0.05% or less, Si: 0.5% or less, Mn: 0.01% to 0.5%, P: 0.04% or less, S: 0.01% or less, Cr: more than 16.0% and 18.0% or less, Ni: more than 4.0% and 5.6% or less, Mo: 1.6% to 4.0%, Cu: 1.5% to 3.0%, Al: 0.001% to 0.10%, and N: 0.050% or less such that Cr, Cu, Ni, and Mo satisfy a specific relationship and (C+N), Mn, Ni, Cu, and (Cr+Mo) satisfy a specific relationship, having a microstructure containing a martensite phase and 10% to 40% of ferrite phase on a volume fraction basis, where the ferrite phase has a length of 50 μm from the surface in the thickness direction and the proportion of the ferrite phase intersecting a plurality of virtual line segments aligned in a row at a pitch of 10 μm in the range of 200 μm is more than 85%, and having a yield strength of 758 MPa or more. It is mentioned that a stainless steel tube or pipe for oil country tubular goods is thereby produced having excellent corrosion resistance in high-temperature environments and having excellent SCC resistance at normal temperature.
Along with recent development of oil wells, gas wells, and the like in severe corrosive environments, a steel tube or pipe for oil country tubular goods has been desired to have high strength and excellent corrosion resistance, where excellent carbon dioxide gas corrosion resistance and excellent sulfide stress corrosion cracking resistance (SCC resistance) and sulfide stress cracking resistance (SSC resistance) are ensured in combination even in severe corrosive environments containing CO2, Cl− and, furthermore, H2S, at high temperatures of 200° C. or higher. However, there is a problem that the SSC resistance in high H2S partial pressure environments has not yet been ensured sufficiently by even the technologies described in Japanese Unexamined Patent Application Publication No. 2005-336595, Japanese Unexamined Patent Application Publication No. 2008-81793, International Publication No. WO 2010/050519, and International Publication No. WO 2010/134498.
It could therefore be helpful to provide a high-strength stainless steel seamless tube or pipe for oil country tubular goods, having high strength and excellent corrosion resistance, where excellent carbon dioxide gas corrosion resistance, excellent sulfide stress corrosion cracking resistance, and excellent sulfide stress cracking resistance are ensured in combination even in the above-described severe corrosive environments, and a method of manufacturing the same.
In this regard, hereafter the term “high strength” refers to a yield strength: 110 ksi (758 MPa) or more. Also, hereafter the term “excellent carbon dioxide gas corrosion resistance” refers to that a corrosion rate of 0.125 mm/y or less when a test is performed by soaking a specimen in a test solution: 20-percent by mass NaCl aqueous solution (solution temperature: 200° C., CO2 gas atmosphere at 30 atm) held in an autoclave for a soaking period of 336 hours. Also, hereafter the term “excellent sulfide stress corrosion cracking resistance” refers to when a test is performed by soaking a specimen in an aqueous solution in which acetic acid+Na acetate is added to a test solution: 20-percent by mass NaCl aqueous solution (solution temperature: 100° C., atmosphere of CO2 gas at 30 atm and H2S at 0.1 atm) to adjust the pH to 3.3, held in an autoclave for a soaking period of 720 hours while an applied stress of 100% of the yield stress is applied and cracking does not occur in the specimen after the test. Also, hereafter the term “excellent sulfide stress cracking resistance” refers to when a test is performed by soaking a specimen in an aqueous solution, in which acetic acid+Na acetate is added to a test solution: 20-percent by mass NaCl aqueous solution (solution temperature: 25° C., atmosphere of CO2 gas at 0.9 atm and H2S at 0.1 atm) to adjust the pH to 3.5, held in an autoclave for a soaking period of 720 hours while an applied stress of 90% of the yield stress is applied and cracking does not occur in the specimen after the test.